Perchlorate salts are substances that affect the thyroid gland, which controls metabolic functions of adults and also promotes body growth of children. In recent years, a series of cases have been reported in which perchlorate ions have been detected at high concentrations in soil or water. Further, since the perchlorate ion (ClO4−) exhibits high solubility in water and is one of the anions that are least likely to interact with a cation, it is difficult to recover perchlorate ions from an aqueous solution as a precipitate or the like. In consideration of the influence of perchlorate ion on tap water, surveys on the concentration of perchlorate ions in rivers have recently been conducted (see, for example, the Journal of the Society of Environmental Instrumentation Control and Automation (EICA), 2006, Vol.11, No. 3, p. 215-218).
As a technique of eliminating a perchlorate salt from a liquid waste contaminated by the perchlorate salt (or perchlorate ions), a method of concentrating a perchlorate salt solution and adding KCl to the concentrated perchlorate salt solution to generate potassium perchlorate (KClO4), then cooling the resultant to crystallize potassium perchlorate, is known (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 9-504472). Further, a water treatment system for eliminating a perchlorate salt using a resin is known (see, for example, JP-A No. 2004-346299 and “NEDO Foreign Report, No. 946, 2004. 12. 15).
Further, compounds that form a capsule so as to enclose ions of various kinds are known (see, for example, J. Am. Chem. Soc., 2003, Vol. 125, No. 28, p. 8595-8613). Such a compound has a structure capable of readily forming a capsule backbone irrespective of the type or size of the ion to be incorporated in the capsule.